A conventional flat tube for a heat exchanger is disclosed in JP2004-293988A. The flat tube is manufactured by laminating a first member and a second member in a manner that both width side rims of the first member are attached on outsides of both width side rims of the second member. The first member and the second member are manufactured by deforming metal plates into narrow gutter shapes. The manufactured flat tube has an outer surface on which stepped differences are formed by exposing end faces of the width side rims of the first member. On the width side rims of the first member, expanded portions expanded outwardly by a thickness of the plate are formed to fill up the stepped differences. Therefore, only on both longitudinal ends, the flat tube has a smooth outer profile where no stepped differences formed on an outer surface.
When manufacturing a heat exchanger, the longitudinal end of the flat tube is inserted into an insertion hole formed on a header and joined by brazing thereon. The both longitudinal ends of the flat tube may be inserted in a pair of headers. Before brazing, the longitudinal ends of the flat tube inserted in the tube insertion hole may be flared in order to improve contact condition between the flat tube and the header.